A private doctor recommended the pilot who crashed a Germanwings jet into the Alps last year should be treated in a psychiatric hospital two weeks before the disaster, a French investigative report says.

The unidentified private physician was one of a number of doctors consulted by Andreas Lubitz as he wrestled with symptoms of a "psychotic depressive episode" that started in December 2014 and may have lasted until the day of the crash, the report said.

Prosecutors believe the 27-year-old pilot, who had a history of severe depression, barricaded himself into the cockpit and deliberately propelled his Airbus jet into a mountainside on March 24, killing all 150 people on board.

In its final report, France's BEA air accident investigation office said Lubitz had begun to show symptoms that could be consistent with a psychotic depressive episode in December 2014 and consulted several doctors over the following months, none of whom alerted aviation authorities or his employer.

Prosecutors have found evidence that Lubitz, who also had eyesight problems and may have feared losing his job, had researched suicide methods and concealed his illness from his employer, sparking a debate on supervision and medical secrecy.

None of the doctors who treated Lubitz, who had been prescribed anti-depressants and sleeping aids, agreed to speak to French or German crash investigators, the agency said.

"In Germany and in France, doctors are very attached to this notion of medical secrecy, but I hope there will be some moves there," BEA director Remi Jouty told a news conference.

The BEAcited a "lack of clear guidelines in German regulations" on when a threat to public safety outweighs the requirements of medical confidentiality.

"Clearer rules are needed to establish when it is necessary to lift medical confidentiality," investigator Arnaud Desjardin said at the launch of the BEA report.

"Several doctors in private practice had the information [that Lubitz] was ill.

"This information was not passed on to aeronautical authorities or to his employer Germanwings."

Despite being seen so many times by doctors, under German law none was able to alert his employers to his state of mind and he was allowed to continue flying.

Lubitz had been flying on a medical certificate that contained a waiver because of a severe depressive episode from August 2008 to July 2009.

The waiver stated that the certificate would become invalid if there was a relapse into depression.

The BEA urged European authorities to carry out more research on the incapacitation of pilots, particularly where psychiatric issues are involved, and to tighten the rules for follow-up checks when pilots with a history of psychiatric problems are declared fit to fly.

Germany's pilot union Cockpit welcomed the French investigators' recommendations, calling them a "balanced package of measures".

"The safety recommendations ... form a balanced package of measures to prevent a repeat of such disasters," said Markus Wahl, spokesman for the union, calling for it to be "implemented in its entirety".

However, Alistair Rosenschein, a former pilot and aviation consultant based in London, said he held the doctors involved in Lubitz's treatment responsible.

"If you decide to work in a safety related industry then you have to forego that confidentiality and it would really be up to the doctor to decide whether or not this impacts on safety to the wider public or not," he told ABC NewsRadio.

"In this case, 150 people lost their lives because the German doctors decided not to inform the airline of the aviation doctors.

"It's outrageous and I hold the doctors — all 41 of them — responsible for this."